Guanyl nucleotide-binding (G) proteins couple agonist interaction with cell surface receptors to an intracellular enzymatic response. In the adenylate cyclase system, inhibitory and stimulatory effects are mediated through the guanyl nucleotide-binding proteins, Gi and Gs, respectively. In the visual excitation complex, the photon receptor rhodopsin is linked to its effector, cGMP phosphodiesterase, through transducin. Bovine brain contains another G protein, Go. The G proteins are heterotrimers of Alpha, Beta, and Gamma subunits; the Alpha-subunits catalyze receptor-stimulated GTP hydrolysis. The interaction of GoAlpha with the BetaGamma subunits and rhodopsin reconstituted in phosphatidylcholine vesicles was examined. The GTPase activity of GoAlpha purified from bovine brain was stimulated by photolyzed, but not dark, rhodopsin and was enhanced by bovine retinal TBetaGamma or by rabbit liver GBetaGamma. GoAlpha in the presence of GBetaGamma is a substrate for pertussis toxincatalyzed ADP-ribosylation; the modification was inhibited by photolyzed rhodopsin and enhanced by GDPBetaS. ADP-ribosylation of GoAlpha by pertussis toxin inhibited photolyzed rhodopsin-stimulated but not basal GTPase activity. It would appear from this and prior studies that GoAlpha is similar to TAlpha and GiAlpha; all three exhibit photolyzed rhodopsin-stimulated GTPase activity, are pertussis toxin substrates, and functionally couple to TBetaGamma. Monoclonal and polyclonal antibodies against G protein subunits have been prepared and characterized. Some of these have effects on function and some have been useful for identification of G proteins in tissues.